This invention generally relates to control systems and methods for three-phase pulse-width-modulated alternating-current voltage regulators (also referred to as AC choppers). More particularly, the present invention relates to a space vector modulation control system and method used with a three-phase pulse-width-modulated alternating-current voltage regulator.
A number of applications, such as air conditioning or refrigeration applications, utilize multi-phase induction motors. The starting, or inrush, current for multi-phase motors tends to be several times the rated full-load current. This high inrush current may have many detrimental effects on the equipment and the power system in general, as well as the economics of power usage. By means of example only, drawing high inrush current over a long power line may cause the voltage to essentially collapse, leaving insufficient voltage for equipment to operate with. Furthermore, with high motor inrush current, other customers along the same power line may experience undesirable voltage fluctuations during the start of the motor. To discourage this situation, power companies sometimes impose penalties if a customer""s starting or inrush currents are excessive. This is particularly true in the regions with xe2x80x9cweakxe2x80x9d power grid, e.g. in Europe. Thus, it is desirable to minimize the current drawn by a multi-phase induction motor during starting.
Several known methods exist, which allow for the reduction of induction motors"" inrush current. Use of an autotransformer is one known methodology for achieving lower motor starting currents. Autotransformers, however, are relatively inflexible, in that the turns ratio of an autotransformer is established up front and remains fixed by the design of the components. Another approach employs the use of series elements such as inductors, resistors, and the like, to limit starting current. The latter approach, however, requires significantly higher line currents than autotransformer starters to provide the same amount of torque in the motor. Yet another approach consists in employing the so-called wye-delta motor starters. This type of equipment configures the connection of induction motor winding in a different manner during the motor start-up than during the regular motor operation. This allows the motor to start with a reduced inrush current.
The above methods for achieving reduced motor inrush currents can all be characterized as electro-mechanical methods. They require a set of electro-mechanical contactors in order to alter the connection of an induction motor to the power line. This altering of connection further results in a reduced voltage being applied to each of the motor""s windings, which in turn results in reduced inrush currents. Electro-mechanical contactors have the disadvantage of being expensive and prone to reliability problems due to wear and tear. In addition, their transitions can cause voltage or current spikes with potentially damaging effects to the system.
The problems associated with electromechanical starting methods for induction motors can be avoided by employing electronic (or solid-state) starting methods. Electronics motor starters reduce the voltage supplied to an induction motor during its startup by means of a power electronics converter. One such converter technology employs thyristors, also called silicon-controlled rectifiers (SCRs). SCRs are semiconductor switches that can be turned on by means of an electronic signal. However, they cannot be forcefully turned off, i.e. they can be turned off only if the current through them naturally extinguishes itself. In a typical SCR-based motor starter, two SCRs are back-to-back connected between each of the motor""s phases and the power line. During the motor start-up, the SCRs are turned on only once in every line cycle, and this is done in a delayed fashion, so that the motor is actually connected to the power line for only a portion of each line period. This results in a reduced voltage being applied to the motor, and therefore a reduced inrush current being drawn from the power line. The amplitude of the fundamental voltage being supplied to the motor is controlled by the time instant when an SCR is turned on within a line cycle. This type of control is usually referred to as phase control.
With no electromechanical contactors needed, SCR-based solid-state motor starters represent an improvement over electromechanical starters in terms of reliability and cost. However, SCR-based electronic starters have the disadvantage of distorting motor""s current and voltage waveforms during the start-up. In addition, they offer no possibility for improving the power factor (power factor is intended as the phase displacement between the fundamental component of voltage and current at the line terminals feeding the motor starter). A good power factor is generally a desired feature in any electrical system.
Alternatively, pulse-width-modulating (PWM) alternating-current (AC) voltage regulators can be used for starting large induction motors, as they allow for a significant reduction in inrush current and provide better quality motor current and voltage waveforms during start-up than SCR-based motor starters. Similarly to SCR-based technology, a PWM AC voltage regulator includes a power electronics converter capable of supplying an output voltage of a fixed frequency, but at a variable magnitude, to AC loads. PWM AC voltage regulators differ from phase-controlled SCR-based AC voltage regulators in that, with PWM AC voltage regulators, the switching of power semiconductors occurs at a frequency many times higher than the input line frequency (usually equal to 50 or 60 Hz). Such high rate of semiconductor switching can be achieved with modern power semiconductors with full turn-on and turn-off capability, such as, for example, insulated gate bipolar transistors (IGBTs). The control of the fundamental amplitude of the output voltage of a PWM AC voltage regulator is achieved through the control of the width of the pulses of which the output voltage waveform in such a regulator consists. A single-phase PWM AC voltage regulator circuit is described in U.S. Pat. No. 5,923,143 to Cosan et al. entitled xe2x80x9cSolid State Motor Starter with Energy Recovery.xe2x80x9d
PWM AC voltage regulators, when used for starting of induction motors, have several advantages compared to SCR-based motor starters. First, they are able to start a motor with a smaller fundamental component of line inrush current. Typically, if an SCR-based motor starter requires an inrush current equal to 45% of motor""s locked-rotor current (LRA), a PWM AC voltage regulator used with the same motor shall require around 20% of LRA. Second, PWM AC voltage regulators generate better-quality motor current and voltage waveforms during the start-up. This results in lower pulsating torque produced by the motor, which, in turn, benefits the motor""s mechanical driveline. Finally, PWM AC voltage regulators offer the possibility for power factor correction.
Practical implementation of three-phase voltage regulators requires semiconductor switches that can conduct, or block, electric current flow in either direction in a fully controllable manner (also referred to as four-quadrant switches). No such single semiconductor device exists nowadays. Therefore, four-quadrant switches are implemented as a combination of at least two two-quadrant switches (i.e. switches that can fully control the current flow in one direction only). Examples of two-quadrant semiconductor switches are bipolar junction transistors (BJTs), gate turn-off thyristors (GTOs, insulated gate bipolar transistors (IGBTs), etc.
Control of a three-phase PWM AC voltage regulator may be accomplished in different ways. Two possible methods are described in A. Mozdzer and B. K. Bose, xe2x80x9cThree-Phase AC Power Control Using Power Transistors,xe2x80x9d IEEE Transactions on Industry Applications, Vol. 1A-12, No. 5, September/October 1976, pp. 499-505 and in S. A. K. Bhat, xe2x80x9cDigitally-controlled multiple-pulse-width-modulated AC chopper for power control,xe2x80x9d International Journal of Electronics, Vol. 51, No. 1, 1981, pp. 45-56. Both control methods are developed for four-quadrant switches implemented with BJTs. They are both dependent on the load power factor, which makes them unsuitable for starting of induction motors (during the start-up, the power factor of a typical induction motor changes form a low value to a high value). Under certain operating conditions, the described methods result in distorted output voltage and current waveforms. In addition, they do not address the issue of PWM AC voltage regulator losses and power factor.
It is desirable, therefore, to provide an improved method and system for control of a three-phase PWM AC voltage regulator. It is further desirable to achieve a variable output voltage with better harmonic content and with lower switching losses. Additionally, it is desirable to improve the power factor by permitting the output voltage to be shifted in phase with respect to the input voltage.
The present invention represents improvements over the known methods and provides a system and method for controlling a PWM AC voltage regulator that is not dependent upon the load power factor and does not result in distortion of output current and voltage waveforms. Rather, by using space vector modulation, it is possible to control a PWM AC voltage regulator to obtain a variable output voltage with better harmonic content and lower switching losses. Further, this method and system permit the output voltage to be shifted in phase with respect to the input voltage thereby improving the power factor.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be appreciated by one of ordinary skill from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in this application and the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, there is provided a pulse-width-modulated alternating-current voltage regulator system to provide a regulated AC voltage from a three-phase voltage source to a three-phase load, such as, for example, a three-phase induction motor as used in air conditioning and refrigeration applications. Between the three-phase voltage source and the three-phase load, the system includes six four-quadrant switches. The first switch is connected between one terminal of the voltage source and one terminal of the load. The second switch is connected between a second terminal of the voltage source and a second terminal of the load. The third switch is connected between a third terminal of the voltage source and a third terminal of the load. The fourth, fifth, and sixth switches are shunt switches between the first, second, and third switches. More particularly, one end of the fourth switch is connected to the junction between the first switch and the first terminal of the load. The other end of the fourth switch is connected to the junction between the second switch and the second terminal of the load. One end of the fifth switch is connected to the junction between the second switch and the second terminal of the load; the other end of the fifth switch is connected to the junction between the third switch and the third terminal of the load. Finally, one end of the sixth switch is connected to the junction between the third switch and the third terminal of the load. The other end of the sixth switch is connected to the junction between the first switch and the first terminal of the load.
The system further includes sensors connected to each phase of the three-phase voltage source to sense the voltage of each phase at any given time. The system also includes a processor for receiving information from these sensors about the input line voltage and for driving the switches based on the received information, as well as predetermined values and software placed in the memory of the processor.
Further, there is provided a method for controlling a three-phase pulse-width-modulated alternating current voltage regulator in a system including six four-quadrant switches. Input and output three-phase voltages are represented by means of space vectors in a two-dimensional voltage plane. The input space vector, representative of the three-phase input line voltages, has a constant magnitude (typically equal to the peak value of the line voltage sinusoid) and rotates around the origin of the voltage plane with a speed equal to the line frequency. A reference output space vector is created based on the desired magnitude and phase of fundamental output voltage. Typically, during the motor start-up, the reference output space vector is smaller in magnitude than the input space vector so that a reduced voltage is provided to the motor. If no power factor correction is desired, the reference output space vector is in phase with the input space vector. Otherwise, there may be a phase displacement between the two. A set of active switching states and a set of zero switching states based on the state of the six four-quadrant switches is provided. Each active switching state is associated with two of six possible output space vectors, representative of the three-phase output voltage of a PWM AC voltage regulator. Each zero switching state is represented by a point in the origin of the voltage plane. The six output space vectors delineate the voltage plane into six sectors. The sensing of input voltages makes it possible to determine the position of the input space vector in the voltage plane. Next, based on the desired phase displacement (with respect to the input space vector) of the reference output space vector, the sector in which the reference output space vector lays at a given time is identified. The two output space vectors bordering the identified sector are then chosen from the set of six possible output space vectors. The active switching states corresponding to the chosen output space vectors are identified next. An appropriate zero switching state is then selected from the provided zero switching states . Finally, by sequencing through the two identified active switching states and the identified appropriate zero switching state at a frequency (referred to as the converter""s switching frequency) many times greater than the line frequency, the desired fundamental output voltage is obtained. As the position of the reference output space vector changes from one sector to another, different (active and zero) switching states are applied to provide the desired output voltage.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.